1. Field of the Invention
The present invention relates generally to endodontic instruments and, more particularly, to root canal files or reamers used in the cleaning of material present in the root canal of a human tooth and for enlarging and shaping the root canal so that it may be prepared for filling and also to the materials necessary to dry, fill and restore the prepared channels.
2. Description of the Related Art
A relatively common but difficult dental procedure is the cleaning, shaping, and filling of the root canal of a patient's tooth. In the performance of a root canal procedure, a hole is first cut in the crown or exposed portion of the tooth, typically either in the biting surface of the tooth, for posterior teeth, or in the side of the tooth on the interior of the jaw for incisor teeth. Small endodontic instruments known generally as root canal files are then used to clean out the material present in the root canal, and to impart a tapered shape to the root canal so that filling material may be inserted into the root canal to seal it. An example of such an instrument, also called a broach, is shown in U.S. Pat. Des. No. 250,544 of Leonard.
Two types of instruments are in general use as root canal files, namely the K-type instrument and the Hedstrom instrument. The K-type instrument is an axially twisted and tapered, triangular or square cross-sectional shaft providing three or four spiral cutting edges along the tapered shaft and a conical tapered tip on the end thereof. An example of a K-type file is disclosed in U.S. Pat. No. 1,307,446 of Kerr. K-type files have recently come to be manufactured with lathe-cut flutes as well. The Hedstrom-type instrument is a lathe-cut file having a round tapered shaft with one, two, or three spiral cutting flutes machined into the shaft all the way to the tip. The main difference resulting from the construction of the two types of files is that the K-type file will cut in either rotational direction, or when moved up and down, while the Hedstrom-type file will cut best when moved up and down in the root canal.
When a root canal is being cleaned and shaped, a series of files having increasing diameters is used to gradually enlarge the root canal. The files are held between the thumb and forefinger of one hand by the dentist. Each file in a set of the known prior art has an identical taper from one end to the other. For example, in a typical K-type file set the taper is 0.32 millimeters on every file over the standard 16 mm length of cutting flutes, or 0.02 mm of taper/mm of flute length. This taper is sometimes referred to as a standard ISO (International Standards Organization) taper. Although these file sets have identical tapers, they come in a number of sizes. The size number characterizing the file is the diameter of the file at the tip in hundredths of a millimeter, and the diameter of the file at the large end is thus 0.32 millimeters greater than this tip diameter. A complete set will include sizes 06, 08, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 70, 80, 90, 100, 110, 120, 130, and 140, while sizes 08-60 will typically be used. Some manufacturers make certain half-sizes, or off-standard sizes.
Hedstrom-type instruments similarly come in sets of increasing size, typically from 0.10 to 1.40 millimeter tip size, flute pitch angle which increases as the instrument tip is approached, thereby decreasing the depth of the flute spaces and increasing the mass of metal in the more fragile tip region. While this feature does add strength to the tips of smaller ground-flute instruments, it creates file tips which are too stiff and inefficient in the larger sizes.
Unfortunately, even twisted-flute #15 files are often not strong enough to withstand the tremendous apical forces which dentists bring to bear when attempting to negotiate calcified canal orifices. As they are pressed into the pulp chamber floor in an attempt to get a "catch" in a calcified canal orifice, these ISO tapered #15 files often buckle, requiring their disposal and the use of yet another #15 file. It is typical that dentists will destroy 10-20 #15 files before they can sneak to the end of these difficult canals.
Another problem countered when using negotiation files of small diameters is the relatively large jump in tip diameter between #10 files and #15 files, a change of 50%. Schilder has addressed this problem in his U.S. Pat. No. 5,017,138 which describes ISO tapered files with tip diameters which increase proportionally in size, providing a series of files with tip sizes 0.10, 0.126, and 0.067 mm in diameter. Maillefer manufactures a non-proprietary file set with half sizes in these smaller instruments, providing clinicians with sizes 0.01, 0.125, 0.15, 0.175, etc., so that progressing between sizes #10 and #15 files is easier.
Root canals are seldom straight and there is always the possibility of causing irreversible damage to roots during shaping procedures, depending on the thinness of the root and severity and with 0.15-0.60 millimeter tip sizes being most commonly used. Both the K-type and Hedstrom files manufactured to ISO standards, whether twisted or lathe-cut, have flute pitches and frequencies which vary little or none in some sizes (large), but quite a bit in other sizes (small). This variation in flute angle or pitch is most perpendicular to the long axis of the file near the shank end, changing to a lessor angle (more in line with the long axis) as the flutes approach the file tip. Only one file has flute angles which vary in the opposite manner, a file disclosed in U.S. Pat. No. 4,674,979 by Jacklich, an ISO-tapered file.
The reason for Jacklich's file design is that ground-flute instruments are not strong enough or stiff enough in the smaller sizes. Ground-flute instruments have their flutes cut into tapered wire blanks which are round in cross-section. This results in the flutes being cut across the grain of the metal, thus achieving less strength than twisted-flute files. Twisted-flute files are formed by first grinding a tapering cross-sectional shape that is triangular, square, or rhomboidal down the length of the wire, in line with the metal grain. Then this tapering wire blank is twisted, causing the corners of the tapered wire blank to become spiral cutting flutes. Because of the twisting process, the metal grain is thus aligned with the helical cutting flutes.
Although stainless steel ground-flute files are of acceptable strength in ISO sizes #20 and larger, they are of inadequate strength in size #'s 06, 08, 10 and 15. In these smaller sizes only the twisted-flute instruments are stiff enough, with a short #15 file being typically used to initially penetrate coronally occluded canals. Jacklich has attempted to circumvent this problem in his ground-flute Hedstrom-type files by creating a location of the root curvatures. If too large a file is advanced too far into a curved root canal, it may easily cut through the side of the root, which is referred to as a perforation of the root, and usually the tooth must then be extracted.
Another cause of root perforation is the inadvertent introduction of large engine-driven Gates-Glidden or Peezo burs into the middle third of thin, curved roots. Quite often, when these burs are new and sharp, the operator will intend to use one of the larger sizes only at the orifice of the canal but will helplessly watch the bur grab the canal walls and pull itself into dangerous depths in the root.
While perforation is probably the worst outcome of mistakes in shaping procedures, there is a more common problem in near-perforations and root weakening caused by overzealous widening of the canal preparation. It is well documented in the endodontic literature that adequate shape in the cervical two-thirds of the canal preparation is mandatory to accomplish adequate cleaning of the canal, to provide necessary control of instruments in the delicate apical regions of the canal, and to effectively obturate the whole root canal space. However, it is difficult to determine the fine line between creating adequate access and dangerous over-instrumentation, as all of these procedures are accomplished in microscopic root canal systems that are hidden from direct view.
Furthermore, if the tip of the file does not follow the curvature of the canal and bores a passage branching out from the root canal, which is referred to as ledging, surgical correction of the problem is often necessitated. It is thus apparent that the art of root canal shaping is one which requires great skill to prevent damage to the tooth and to create a tapered canal preparation conducive to ideal filling of the canal.
The technique used with a conventional set of files having identical tapers to clean and shape the root canal is referred to as the "step-back" technique. A series of file sizes from 08 to 60 (12 instruments) are introduced into the canal from smallest to largest with each successively larger file being used further back from the end of the canal. Additionally, between 4 and 6 sizes of Gates-Glidden or Peezo burs are similarly used in this step-back manner comprising a total of 16 to 18 instruments.
This technique is, at best, a difficult and time-consuming method as the dentist must indirectly gauge the rate of taper in the preparation by the distance interval of step-back of the progressively larger instruments as they fit further back from the canal terminus. As only the ISO taper of 0.02 mm/mm is currently available in standard sizes to dentists in files, irrigation cannulas, condensation pluggers, heat carriers and injection needles, paper points, filling material, and restorative posts, the current shift to canal preparations of greater taper has created great difficulties for dentists who want to enhance their root canal shaping objectives. The skills required, with these relatively non-tapered instruments and materials, to create conservative but adequately tapered shapes in root canals and to easily and ideally fill them usually comes only after treating hundreds of clinical cases.
In my prior U.S. Pat. No. 4,836,780, I disclosed a solution to this shaping problem which involved a series of tapered files with Hedstrom flutes to be used in a push-pull motion, the files having a SAFE EDGE.TM. or non-cutting surface along one longitudinal side of the instrument. While these variably tapered instruments can create continuously tapered root canal preparations, they are extremely difficult to pull out of the canal by hand because the whole length of the cutting flutes is engaged in the canal, as opposed to standard ISO Hedstrom files which are relatively untapered, engaging less canal length during shaping, and therefore easier to use in a push-pull fashion.
In his U.S. Pat. No. 4,536,159, Roane disclosed a new, more effective method of manipulating ISO-tapered K-type root canal files, wherein the file is lightly rotated in a clockwise direction to thread the file into the canal, after which, with apical force applied, the file is counterclockwise rotated to cut and plane the canal walls. Because the clockwise direction of the flutes causes the file to back out of the canal when it is counter-rotated, the file tip tends to center in the canal just before it cuts, if the apical force being applied to the handle is great enough to keep the file in place. In addition, Roane determined that straight ISO-tapered files can be used in curved roots without risk of perforation as long as rotary cutting motions are used and push-pull motions are avoided. Thus, aggressive rotary shaping of curved canals with unbent ISO-tapered K-type files can be accomplished without ledging or perforation of the canal walls.
While this filing motion would aid the use of variably-tapered K-type shaping files with their challenging full-length engagement of the canal walls, it became apparent during my testing that as files were given greater tapers, they became more likely to perforate curved roots, due to the increasing stiffness attendant with the larger shank diameters of the more tapered files in the series. Additionally, the standard "hour-glass" file handle disclosed in Roane's patent, while not ideal for rotary cutting techniques with ISO-tapered files, is actually inadequate, due its shape (designed for push-pull filing motions) and narrow diameter, for rotary cutting with files of greater taper. The SAFE EDGE.TM. feature, while eliminating lateral root perforation when using push-pull filing motions, by definition excludes the use of rotational cutting motions, as the safe-edge would then move to all surfaces of the canal, thereby negating the safety feature's function in curved roots.
Other prior art to be mentioned here is that relating to the non-standardized nature of conventional irrigation cannulas, paper points, gutta percha points, obturation carriers, obturation pluggers, heat carriers, backfillers, and restorative post systems when used in the creation and treatment of tapered root canal preparations. Existing irrigation cannulas are parallel-sided, with different designs of opening at their tips. These are used to wash the inside of root canal systems with solutions with the intention of debriding, disinfecting, and cleansing the canal space prior to sealing it. Unfortunately, these irrigating cannulas are limited in their ability to clean the ends of root canal systems as they are so small in diameter. Research to date shows that irrigating cannulas are only effective to the extent that they can penetrate the canal, usually to midroot, leaving the important apical third of the canal unaffected.
Dentists are able to overcome this problem by the use of small patency files which are used to clear debris from the end of the canal, but which also displace irrigant from this apical region of the canal during its use, allowing fresh irrigant back into the apical region after the file's withdrawal. As the files are relatively untapered (ISO 0.02 mm/mm) this irrigant exchange is limited.
When the canal has been cleaned and shaped, it is dried with absorbent paper points of varying size and taper. In canals which have been tapered with burs and files in a step-back fashion, it is often necessary to use several different sizes of paper points to dry the whole length of the canal, small paper points apically, larger paper points for the middle of the canal, and sometimes even larger paper points for the cervical region of the canal. Furthermore, as the tapers of the paper points seldom match the taper of the canals, there may be moisture left on parts of the canal walls after drying procedures.
There are several methods of sealing root canal systems, most of them using a rubber-like material called gutta percha. When used as a tapered gutta percha cone, this material is compacted into the canal with pluggers, or the gutta percha material may be placed as a coating on an obturating carrier, which is warmed to soften the gutta percha and placed in the canal, with the carrier compacting the material into the canal space.
Currently, the shapes of filling materials and obturating carriers do not match the tapering shapes of prepared canals either. When the step-back technique of canal shaping is used, the final shape of the canal preparation can only be discerned indirectly, by the increments that each larger instrument fits further back from the terminus of the canal, a difficult skill learned only after much experience. As the prepared taper is often obscure to the clinician, it is likewise difficult to pick an appropriately tapered gutta percha point or obturating carrier with which to seal the canal. If the selected obturating device or gutta percha point is too tapered, they will bind in the canal short of its terminus, causing the crucial apical seal to be inadequate, allowing leakage and failure of the endodontic treatment. If the obturating carrier or material is too narrow, little hydraulic pressure will be exerted on the filling materials in the cervical two-thirds of the canal during condensation procedures, and lateral or accessory canals in that region of the canal may not be sealed, again increasing the chance for failure of treatment.
While there are many techniques of filling root canals, it is generally recognized in the field of endodontics that those methods which warm and soften the gutta percha filling material, allowing it to be thoroughly compacted into all the nooks and crannies of root canal systems, are superior to those techniques which do not thermoplasticize the gutta percha prior to condensation.
The classic technique was described by Schilder in 1969, vertical condensation of warm gutta percha. In this technique an appropriately tapered gutta percha cone is fit and cemented in the prepared canal, after which a flame-heated or electrically-heated gutta percha heat carrier is used to sear off the gutta percha cone at the orifice level of the canal. By pressing the softened gutta percha into the canal with an appropriately sized vertical condensation plugger, the first wave of condensation is initiated, filling any lateral canals present in that region in the primary canal. The heat carrier is then reintroduced into the canal, penetrating the gutta percha several millimeters, heating the apical mass, and removing a portion of it so that the next wave of condensation may occur deeper in the root. These heating and compacting cycles continue until the final wave of condensation which ends 5-7 mm from the canal terminus.
Typically it takes 3-7 waves of condensation to reach this end point. At this point the clinician must either place a retentive post in the coronal canal space or backfill it with gutta percha. Backfilling can be done by heating 3-8 small pieces of gutta percha and sequentially packing them into the canal, or by syringing alloquates of pre-heated gutta percha from a gutta percha gun and compacting them with pluggers. Downpacking with multiple waves of condensation and backfilling in the manners described require at least 7-10 different instruments, fairly extensive training of the dentist and chairside assistant, and 15-30 minutes of clinical time. Furthermore, these condensation pluggers and heat carriers lack a correlating mechanism to match their sizes to the taper of the canal preparation.